An electromechanical acoustical transducer is a device that converts electrical signals into acoustical signals where a given electrical input mechanically imparts a specific velocity displacement to a mechanical diaphragm whose velocity displacement produces an acoustical signal conveyed to one or more listeners through a transmission medium consisting of a variety of molecular gases commonly called “air.” In free space, this assembly of molecular gases (air) shows specific mechanical characteristics as a consequence of molecular cohesion. When approaching a boundary, molecular adhesion of the gas molecules to the boundary surface changes the mechanical characteristics of the gas.
Extended range transducers fail as perfect electrical to acoustical transducers in part because of a variety of unintended material vibration modes within the transducer diaphragm material. The presence of a diaphragm material vibration mode produces a frequency dependent change in the acoustic output magnitude that varies from the corresponding frequency dependent voltage or signal magnitude of the input. One form of this diaphragm material vibration mode often occurs just before a given transducer's high frequency roll-off. This phenomenon is often characterized by a sharp rise in output magnitude just before a precipitous fall-off in acoustical output. One example of this is shown in FIG. 6. This combination of sharp rise preceding a sharp fall-off in acoustical output magnitude is an indicator of what is called diaphragm “breakup.”
A variety of devices occupying space on the front side of a cone and within the front side cone volume have been specified. These devices have been variously called diffusers, phase plugs, and compression phase plugs. Generally phase plugs (FIG. 1) have been used with direct radiator acoustical transducers and compression phase plugs have been used with horn type acoustical transducers (FIG. 2).
It is commonly held that both phase plugs and compression phase plugs work according to principles of wave-guide theory. For a direct radiator transducer (FIG. 1) the phase plug operation is described from a ray propagation perspective. A direct radiator phase plug usually has the shape of a cylinder with one end having decreasing radius along a curve until reaching a point at the front side of the plug. The plug 12 is cylindrical until near the front or top edge of the voice coil former and begins its curvature of decreasing radius either slightly below or above this point. This shape of phase plug is usually described as having a bullet shape. Sound generated by the velocity displacement of the cone diaphragm 10 is said to leave the surface of the cone as a ray in a direction perpendicular to the cone surface. This sound ray then strikes the curved portion of the phase plug and the ray is deflected and redirected by interaction with the phase plug's curved surface and is commonly held to thereafter have a direction of propagation parallel to the central axis of the cone.
For a compression type phase plug (FIG. 2), the transducer diaphragm 10 will usually include a dust cap 11 to cover the front edge of the voice coil former and the compression type plug 14 will cover the majority of the cone with a surface profile on the cone side of the plug that follows the cone and dust cap shape profile. There will typically be a complex series of radial slots in the plug. Compression plugs and the slots in the plugs again conform to the principles of wave-guide theory. The function of the slots is commonly held as forming guides that equalize the propagation path length of sound rays emanating from different sections of the cone and dust cap surface such that they leave the exits of the phase plug slots with all rays coincident regardless of point of origin on the varying shape of the diaphragm and dust cap.
While both plug designs have influence upon the performance of the acoustical transducer utilizing them, they are not intended to account for boundary effects and do not produce significant changes in the acoustical output in the range of frequencies considered by this invention. An example of a direct radiator, cone type diaphragm 10 acoustical transducer utilizing a bullet shaped phase plug 12 is shown in FIG. 3. The frequency response produced by the bullet shaped phase plug is shown in FIG. 4. The electrical signal inputted to this transducer has equal energy at all frequencies examined by this test. This response may be compared with the output frequency response shown in FIG. 6. The transducer shown in cutaway side view in FIG. 5 produces the frequency response shown in FIG. 6. This sample has neither phase plug nor dust cap.
As compared to the frequency response performance of the driver of FIG. 5, while there is some improvement in frequency response, the sharp rise and then fall in output magnitude between 10 and 20 kHz is still evident. The wave-guide designed plug of FIG. 3 also causes two small peaks in output magnitude at 6 and 8 kHz and its use may be said to produce a decrease in performance.